Joining of advanced materials such as high temperature metals (e.g., nickel-based superalloys and intermetallics), ceramics, and composites based upon these materials is a challenging technological problem. (See, for example, "Joining and Adhesion of Advanced Inorganic Materials," A. Carim, D. Schwartz and R. Silberglitt, eds., MRS Symposium Proceedings Vol. 314, Materials Research Society, Pittsburgh, Pa. 1993.) For metals, maintaining the desired microstructure and preventing the formation of potentially brittle intermetallic phases are problems. The combination of high strength and low fracture toughness makes ceramics brittle materials, vulnerable to cracking induced by any residual stresses resulting from the joining process. Composites with either metallic or ceramic matrices, typically reinforced with ceramic fibers, whiskers or particles, have good fracture strength and flaw tolerance, but their thermal and mechanical properties are sufficiently different from those of the metals that they are intended to replace that discontinuities at the bonding layer, particularly in thermal expansion coefficient, lead to weak joints. (See, for example, R. D. Watkins in "Engineered Materials Handbook, Volume 4: Ceramics and Glasses," ASM International, Metals Park, Ohio, 1991, p. 478.)
Joining of dissimilar materials is generally difficult because of such discontinuities. Interlayer materials with graded properties (called functionally gradient materials) are sometimes employed to minimize stresses that can lead to joint failure. However, providing the energy needed to heat the interlayer material to joining temperature and holding it at that temperature for the necessary time can lead to degradation of the properties of the materials being joined.
Ceramic or ceramic matrix composite-metal joining is important because the ceramic and composite materials have the potential for increased service temperatures, leading to improved performance, in many aerospace and industrial applications (e.g., turbine combustors and rotors, industrial heat exchangers and chemical process systems). The introduction of the ceramics in the high temperature components or sections of systems will require joining them to metallic components. The simplest way to accomplish this is a mechanical interlock. However, mechanical interlocking requires extensive machining of the ceramic component, which is often very expensive. In addition, wear and friction at the mechanically joined interface, caused by high speed motions and thermal cycling stress during service, may lead to reduction of bond strength and fatigue flaws. Another approach is to heat the interface to high temperature while maintaining good contact, to allow interdiffusion of the materials. While this is a popular technique for joining advanced metals, high temperatures and long times are necessary for strong diffusion bonding of ceramics because of the extremely low diffusivities of most atoms and ions in ceramics at the relevant temperatures.
The most commonly used method for joining metals and ceramics is brazing. Good braze joints have been obtained through the application of a refractory metal paste (e.g., Mo or W) on a ceramic surface, followed by sintering to form a metallized glassy layer on the ceramic. Metals can then be readily brazed to the metallized ceramic. (See, for example, V. A. Greenhut, "Progress in Ceramic Metal Joining and Metallization," Proceedings of TMS Fall Meeting, The Metallurgical, Minerals and Materials Society, Cleveland, Ohio, 1990, p. 103 and J. Intrater, "The Challenge of Bonding Metals to Ceramics," Machine Design, Nov., 1989). Alternatively,brazing can be accomplished in one step by introducing active metals such as Ti, Al, Zr and Hf into brazing alloy compositions. (See, for example, M. Santella, "A Review of Technology for Joining Advanced Ceramics," Ceramic Bulletin, vol. 71, No. 6, American Ceramic Society, Westerville, Ohio, 1992, p. 947.)
While brazing can provide strong metal-ceramic joints, the properties of these joints degrade rapidly at high temperature (e.g., T&gt;1000.degree. C.). For example, active metals have poor oxidation resistance at elevated temperatures. They may continue to react with the mating materials during service at elevated temperature, leading to the formation of intermetallic layers. The embrittlement of the intermetallic phase will then dramatically reduce the bond strength. Glassy phases can also impair strength, toughness and thermal properties of the final assembly under high temperature service conditions.